avcodec/jpeg2000dec: Skip de-quantization of empty areas

Fixes: Timeout (26sec -> 18sec)
Fixes: 13448/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-576903098243481

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/jpeg2000dec.c

@@ -1730,6 +1730,8 @@ static inline void tile_codeblocks(Jpeg2000DecoderContext *s, Jpeg2000Tile *tile
                                     bandpos);
                         if (ret)
                             coded = 1;
+                        else
+                            continue;
                         x = cblk->coord[0][0] - band->coord[0][0];
                         y = cblk->coord[1][0] - band->coord[1][0];